1. Field of the Invention
This invention relates to a driver circuit configuration used with the magnetic deflection yoke of a typical cathode ray tube and, particularly, to a high speed circuit configuration used in the driver channels for a magnetic deflection type of cathode ray tube.
2. Description of the Prior Art
CRT (cathode ray tube) display devices are well known and have been used for many years to display a visual readout of electrical information derived from a multitude of different sources. A magnetic yoke is often employed to create a magnetomotive force on the beam of electrons emitted by the cathode of the CRT to deflect the beam along two independent axes.
Numerous circuit configurations have been proposed for driving the horizontal and vertical winding of the magnetic deflection yoke. In some applications of a CRT, such as a readout device for displaying alphanumeric characters, it is well known that the band pass characteristic of both the vertical and horizontal drive circuits should be as high as possible to obtain optimum character fidelity, i.e. sharp points and edges of the individual characters.
In many driver circuit configurations, the maximum frequency response of the channel is limited by the resonance of the specific yoke, which in turn is a function of the impedance of that specific yoke. Prior art circuit configurations for driving both the horizontal and vertical deflection winding of the magnetic yoke have included the high gain operational amplifier in which the yoke is connected so that it is in a feedback loop of the operational amplifier. In such a circuit configuration, the upper limit of the band pass characteristic of the channel was limited by the yoke resonance, the compensation therefor was to cause the overall channel to be designed to roll off prior to the resonant point of the yoke. By causing the channel bandwidth to roll of prior to the resonant frequency, instability in the channel is prevented by assuring adequate feedback gain and phase margins.
Of course, as indicated hereinbefore, the lower the frequency at which the channel begins to roll off, the greater the degradation of the characters on the faceplate of the CRT so the rate at which information can be displayed on a CRT is directly related to the band pass of the driver circuitry. Further, a smooth roll off of the band pass ability of the channel associated with a magnetic driver circuit is complicated by the fact that in a typical yoke, a null point often occurs at a frequency slightly above the frequency of primary yoke resonance because of the relationship between the stray capacitance and stray inductance. At this null frequency, the effective impedance of the yoke has changed to a minimum value from the maximum which occurred at the frequency of the primary resonance. This rapid change in impedance further complicates the design of a smooth response at the high end of a yoke driver channel by reducing the feedback gain and phase margins. Even at the frequencies beyond the null frequency, the high Q can cause an impedance change which, if uncompensated for, would allow the system to become unstable or oscillate.
As is known, instability at any frequency of a driver circuit for one of the two orthogonal channels will cause the entire channel to be unsuitable for its intended purpose. The circuit design considerations associated with the high frequency end of the driver channel pose the most significant problems. The horizontal winding and the vertical winding of a magnetic deflection yoke are in close proximity to each other resulting in parasitic capacitance therebetween which impacts the operation of each channel individually. For example, it is known that in a yoke for orthogonal deflection if one winding is connected to a conventional feedback driver circuit so that it is essentially grounded, or at least coupled to ground through a small resistance, the yoke impedance characteristic will have a reasonably sharp resonance point followed by a sharp null point. However, if that same second channel is opened so that it is not connected to ground (not the normal case in operation), the parasitic resonance between the two yoke windings changes thereby improving the overall band pass characteristics of the first channel.
Of interest is U.S. Pat. No. 3,307,067 issued Feb. 28, 1967 to P. L. Jachim et al for DYNAMIC BLUE LATERAL CORRECTION SYSTEM. The beam control apparatus disclosed in this patent includes a first and second inductively coupled winding positioned adjacent the pre-convergence path of the beam providing the blue raster. The first winding is connected in series with the deflection yoke to carry saw tooth deflection current. A variable impedance is connected across the second winding to form a control loop so that induced current flowing therein can be varied to control the net flux produced by the corrector. This disclosure does not appear to be concerned with the bandwidth enhancement of a driver channel for orthogonal windings of a magnetic deflection yoke.